1. Field of the Invention
The present invention relates to color photographic materials and particularly to color photographic materials wherein the dye formation efficiency in the color development step is high, photographic properties are not influenced by variation in the pH of the color developing bath and the color images are fast to heat or light.
2. Description of the Prior Art
As magenta image forming couplers (referred to as "magenta couplers", hereinafter), various pyrazolone derivatives have been known. However, these pyrazolone derivatives have low color formation efficiency (ratio of conversion of the coupler into a dye) contained in photographic sensitive materials and so-called 4-equivalent couplers in which coupling active positions are not substituted form only about 1/2 mol dye per mol of the coupler.
To improve color formation efficiency, so-called 2-equivalent magenta couplers have been used in which a substituent is introduced into the coupling active position of the pyrazolone type magenta coupler by which the substituent splits off in the color development step. Examples are disclosed in U.S. Pat. Nos. 3,311,476, 3,419,391, 3,617,291 and 3,926,631. Further, magenta couplers in which a substituent is linked to the coupling position through a sulfur ion are described in U.S. Pat. No. 3,214,437 (a thiocyano group), U.S. Pat. No. 4,032,346 (an acylthio or as thioacylthio) and U.S. Pat. Nos. 3,227,554 and 3,701,783 (heterocyclic thio or arylthio).
The magenta couplers used for color photosensitive materials of the present invention have an alkylthio group at the coupling active position of the pyrazolone nucleus, and are quite different in chemical structure from couplers described in the above described U.S. Pat. Nos. 3,214,437 and 4,032,346. Further, couplers having an alkylthio group of 6 to 22 carbon atoms have been claimed in U.S. Pat. Nos. 3,227,554 and 3,701,783, but there is no description of magenta couplers having an alkylthio group in these patent specifications. The couplers having an arylthio group or a heterocyclic thio group described in U.S. Pat. Nos. 3,227,554 and 3,701,783 have been known as release inhibition type couplers (DIR couplers) in the art, which function to retard the development by interaction of the arylthio group or the heterocyclic thio group released from the coupling active position of the couplers with silver halide in the development step. Couplers of the present invention from which an alkylthio group is released are functionally different from the above-described couplers, because they do not have a release inhibiting function.